This invention relates, especially to the forming of large diameter plastic pipe and fittings having bell ends, The term "fittings" generally includes Tees, Ell's, 30.degree., 45.degree. and 90.degree. elbows, pipe couplings, end caps, wyes, other angles, and the like. The term "bell end" on a plastic pipe or pipe fitting denotes an enlarged end, with respect to the outer diameter of the remaining portion of the pipe fitting, which enlarged end is provided with an annular groove in its inner surface normally adapted to receive an annular resilient gasket. The term "plastic" as used herein, and in the claims, generally denotes thermoplastic material, examples of which are polyvinylchloride (PVC), acrylic butadiene-styrene polymers (ABS), polyethylene and styrenebutadiene polymers.
Pipe fittings are capable of transferring tremendous quantities of fluid at high pressure only if joints between the pipe fittings and the pipe proper are properly sealed. Solvent welding of pipe to the pipe fitting, whether it be ells, 45.degree. bends, tees, eyes, couplings, or whatever, may be employed but have drawbacks in terms of quality control, during assembly, especially in large diameter fittings. The irrigation, water distribution, and sewer/drain industries and other industries are therefore moving to the use of pipe fittings with flared or bell ends wherein annular races are formed at the ends of the pipe fittings, the annular races being adapted to accept annular rubber gaskets for fluid-tight sealing with an adjoining piece of pipe.
The problem in the industry has been one of economically manufacturing, on a mass production basis, bell ends in metal or plastic pipe fittings. Many companies presently machine the recesses or races which, of course, is costly. Others (e.g., Shupe, U.S. Pat. No. 3,474,999) have tried to mold the race at the same time the entire part is made and then, by means of a system of internal metal cores at the race which collapse inwardly, the internal metal cores are removed from the race to then permit removal of the part. This leads to very complicated tooling and also, consequently, great expense.
The patented art also reveals the use of deformable (elastomeric) internal cores, to form the bell ends, e.g., Huck, U.S. Pat. No. 3,377,659, Axelrod, U.S. Pat. No. 2,714,226 and Muhlner, U.S. Pat. No. 3,520,047 and Patent No. 3,507,505. Shupe, supra, explains the disadvantages of using elastomeric mold materials, as internal mold pieces, e.g., the necessity for frequent replacement of such a mold, the difficulty of removal of such molds from the completed race, and the real possibility of permanent deformation of such mold piece itself from its intended shape after frequent usage.
One of the inventors herein, namely Donald Bowker, is generally aware of a process and apparatus, practiced in a country foreign to the United States of America, in which rigid internal axially retractable cores have been utilized to form bell ends in plastic pipe fittings. So far as he is aware, such process and apparatus utilizes a split cavity in which a two-part mold means is placed defining the part to be molded. The two part mold has an outer part defining the external portion of the bell end and has an inner core which is fully retracted after the outer portion of the mold means is retracted. Cooling means to chill both the cavity and the mold means (and thereby the part to be molded) are utilized.
While chilling of the cavity and mold means is of advantage in achieving a relatively high production rate, it is believed that still further economies of production can be realized by utilizing a specific temperature differential within the mold means, per se, and/or by achieving a multiple stage retraction of the inner core of the mold means, rather than by a single stage retraction of the inner core, as in the just-mentioned process of which Mr. Bowker is aware. Further, Mr. Bowker is not aware of the use of a means to achieve a specific temperature differential which includes a high-conductivity metal for the mold piece defining the body portion of the fitting together with a dissimilar low-conductivity metal for the mold pieces defining the bell end portion of the fitting.